1. Field of the Invention
The invention relates to a wedge-type anchor for stranded tension cables.
2. Description of the Related Art
Stranded tension cables are almost exclusively anchored with wedge systems that involve longitudinally segmented wedges with hardened teeth on the inside (e.g. German OS No. 2 720 788). The relatively hard points of the teeth dig into the cable as it is tensioned and create both a frictional and mechanical connection between the cable and the wedge.
The anchoring systems utilized in the present context--for securing prestressed-concrete sections that are employed for example in tanks for storing liquefied gas and are subjected to very low temperatures--encounter problems as a result of the very high notch sensitivity of the cable material even though a number of tension steels with satisfactory strength along their total length even at cryogenic temperatures are available. Notch sensitivity can at very low temperatures lead to premature fracture of the steel. The fracture occurs in the vicinity of the anchor, usually without plastic deformation of the steel along its free length. Thus the safety standard for pretensioned structures that the steel should have satisfactory overall strain (elongation) in the computed fracture state is not complied with. This demand for a plastic strain component can be complied with only when the yield point of the steel is considerably exceeded at cryogenic temperature.
At very low temperatures there is another problem in the design of the cables that are to be anchored. The cable usually consists of six outer wires and one core. The outer wires are in direct contact with the wedge in the vicinity of the anchor, whereas the core is secured only by friction with the outer wires. Transverse pressure is communicated linearly from the outer strands to the core. When the cable is subjected to high tensile load and accordingly higher transverse pressure at room temperature, the wires will deform and increase the contact surfaces. The strain hardening that occurs at very low temperatures prevents or at least inhibits the deformation that develops at room temperature and high tensile load and increases the contact surfaces. The core will consequently slip prematurely and become deprived of support in the accommodation of force. If the core slips completely or almost completely out of the wedge, the anchor will suddenly fail. If the core catches again inside the wedge, the outer strands will be overloaded and will break prematurely.
Wedge-type anchors in which the transverse pressure (compression) and hence the depth that the teeth penetrate to are supposed to increase gradually from the thin end or point of the segmented wedge are state of the art. Thus, in one known wedge-type anchor, the increase in compression from the point of the wedge is generated by preventing the parts at the thin end of the wedge from resting against the wall of the wedge mount (German OS No. 2 720 788). Tapering the bore that accommodates the wedge outward at a section that has a lower angle of inclination than the main section has also been disclosed in this context. To generally obtain a "weaker" grip on the tensioning member of a wedge anchor, it is also state of the art to decrease the slip-resistance of the inner surface of the wedge that is in contact with the member at the thinner end of the wedge (German OS No. 2 357 819). This can be done in particular by decreasing the height and shape of the gripping projections cast onto the inner surface of the wedge toward its thinner end. The gripping projections are, specifically, helical serrations with a pitch that decreases continuously from the thicker to the thinner end of the wedge. They are of course expensive to produce.
Finally, a fastener for stranded tension cables that involves wedge-shaped elements with engagement projections cast onto the surfaces that come into contact with the cable and decreasing as in the aforesaid anchor from the thicker to the thinner end of the wedge (GG Pat. No.549 616) is also known. These projections actually disappear two thirds of the way along the wedge, which is about seven times the total diameter of the cable. The angle of outside inclination of the wedge-shaped elements can be sharper than the angle of inside inclination of an associated sleeve. This type of anchor requires a long wedge to hold the tension member securely. It is accordingly relatively expensive, the more so because the wedge-shaped elements have to be reinforced at their narrow end with an additional continuous ring.
None of these known wedge anchors, however, are designed to comply with the special demands associated with anchors that are to be exposed to cryogenic temperatures.